In most recently published articles, the DOI will be printed with the article itself, usually on the first page somewhere, or in the header or footer.
Article databases, such as PubMed and CINAHL, will list the DOI as part of the bibliographic information of the article. Examples below:
If the DOI isn’t on the article or in the database record, look it up on the website CrossRef: https://doi.crossref.org/simpleTextQuery. This free website allows you to paste all or part of a reference to find the corresponding DOI.